Antibody based cancer therapies have been successfully introduced into the clinic and have emerged as the most promising therapeutics in oncology over the last decade.
Antibody-based therapies for cancer have the potential of higher specificity and lower side effect profile as compared to conventional drugs. The reason is a precise distinction between normal and neoplastic cells by antibodies and the fact that their mode of action relies on less toxic immunological anti-tumor mechanisms, such as complement activation and recruitment of cytotoxic immune cells.
Targets for antibody-based therapies need to have particular qualities, which form the basis for proper discrimination between normal and neoplastic cells. Obviously, a target with either exclusive restriction to tumor cells and entirely undetectable on normal tissues is ideal for the development of efficient and safe antibody therapeutics. In another aspect, a high-level overexpression may be the basis for the therapeutic window and low side effects exemplified by the human epidermal growth factor receptor type 2 (HER-2), which as a result of gene amplification is a good target for the antibody trastuzumab (Herceptin).
Other targets for antibodies which are either already approved or in clinical development for tumor therapy have distinct qualities, which are not based on a numeric overexpression of target molecules on tumor cells. In the case of antibodies to the proteoglycan MUC-1, a peptide repeat epitope in the backbone of the target is underglycosylated in tumor cells and thus altered to its normal counterpart. In the case of antibodies to CD20 (rituximab), CD52 (Campath-1H) and CD22 (epratuzumab), antibody targets have comparable expression levels on tumor cells and normal lymphocytes. Here, the ablation of normal cells by the antibody is tolerable since target-negative stem cells restore the normal lymphocyte repertoire. Other examples of differential accessibility of antibody targets are carcinoembryonal antigen (CEA) and carboanhydrase IX (CA9). Both antigens are expressed on normal epithelia of colon and kidney, respectively. However, radioactively labeled imaging antibodies do distinguish well between tumor and normal tissue, and cytotoxic antibodies are well tolerated. This is most likely due to a restricted expression of CA9 and CEA on the luminal side of normal epithelial tissue when IgG antibodies do not have access. Also antigen epithelial cell adhesion molecule (Ep-CAM) belongs to this category. As a homotypic cell adhesion molecule for epithelial cells it is localized in the intercellular space. Intriguingly, whereas high-affinity anti-Ep-CAM antibodies are very toxic, intermediate-affinity antibodies are well tolerated. This suggests accessibility of the Ep-CAM target on normal cells but also indicates that kinetics of antibody binding may open a therapeutic window.
Eight antibodies have been approved for treatment of neoplastic diseases, most of them, however in lymphoma and leukemia (Adams, G. P. & Weiner, L. M. (2005) Nat. Biotechnol. 23, 1147-1157). Only three mAbs, namely Herceptin, Avastin and Erbitux, address solid cancer types, which account for more than 90% of cancer-evoked mortality. The substantial remaining medical need, the significant clinical benefit approved mAbs have already provided and their considerable commercial success altogether motivated a wave of innovative approaches standing poised not only to develop antibody-based therapies for extended groups of patients but also to improve their efficacy (Brekke, O. H. & Sandlie, I. (2003) Nat. Rev. Drug Discov. 2, 52-62; Carter, P. (2001) Nat. Rev. Cancer 1, 118-129).
One of the challenges to be mastered for the advent of the next generation of upgraded antibody-based cancer therapeutics is the selection of appropriate target molecules, which is the key for a favorable toxicity/efficacy profile.
Current antibodies available for the treatment of solid cancers owing to the expression of their targets on normal tissues do not sufficiently exploit the cumulative power of action modes embedded in antibody molecules. Her2/neu, for instance, the target of Herceptin, is expressed in many normal human tissues including heart muscle (Crone, S. A., Zhao, Y. Y., Fan, L., Gu, Y., Minamisawa, S., Liu, Y., Peterson, K. L., Chen, J., Kahn, R., Condorelli, G. et al. (2002) Nat. Med. 8, 459-465). As a consequence, Herceptin was designed with a reduced immunological potency and cannot be given at the maximum effective dose, because of otherwise unacceptable toxicity. This “blunting of a potentially sharp knife” limits the therapeutic efficacy of Herceptin.
In addition to lack of expression in toxicity relevant normal tissues, robust and high expression on the surface of tumor cells and exhibition of a tumor promoting function are desirable characteristics for an ideal antibody target (Houshmand, P. & Zlotnik, A. (2003) Curr. Opin. Cell Biol. 15, 640-644).
Using an integrated data mining and experimental validation approach for the discovery of new targets for antibody therapy of cancer we identified GT468. GT468 is a placenta-specific gene with no detectable expression in any other normal human tissue. However, it is frequently aberrantly activated and highly expressed in a variety of tumor types, in particular breast cancer. RNAi-mediated silencing of GT468 in MCF-7 and BT-549 breast cancer cells profoundly impairs motility, migration and invasion and induces a G1/S cell cycle block with nearly complete abrogation of proliferation. Knock down of GT468 is associated with decreased expression of cyclin D1 and reduced phosphorylation of AKT kinase. Moreover, GT468 is localized on the surface of cancer cells and is accessible for antibodies which antagonize biological functions of this molecule.
GT468 has several properties that make it a highly attractive target for therapeutic antibodies. Being a differentiation antigen of a cell lineage which appears in the human body only in such an exceptional state as pregnancy, it is as absent from healthy toxicity-relevant tissues as a self-antigen can possibly be. Its high prevalence in a variety of tumor entities would make a broad number of patients eligible for treatment with GT468 targeting therapies. In the case of breast cancer for example, 82% of patients carry this target. Her2/neu, in contrast, the target of Herceptin, the only mAb available for treatment of this cancer type, is overexpressed in only 20-25% of breast cancer patients (Slamon, D. J., Godolphin, W., Jones, L. A., Holt, J. A., Wong, S. G., Keith, D. E., Levin, W. J., Stuart, S. G., Udove, J., Ullrich, A. et al. (1989) Science 244, 707-712). For lung cancer and for gastric cancer, in which GT468 is expressed in 42 and 58% of the cases respectively, there is no approved mAB treatment so far owing to the lack of appropriate targets in these cancer types.
GT468 is druggable by antibodies on living cells and such antibodies may precipitate anti-tumoral effects such as proliferation inhibition. GT468 is involved not only in proliferation but also cell motility, migration and invasion. Most interestingly, all these attributes do not only substantially contribute to the tumor phenotype but are also inherent properties of the human trophoblast, which physiological characteristics are to grow fast and to invade efficiently into uterus tissue. It is expected that mAbs against GT468 can be engineered, which intervene with all these functions at once on top of their potential to mediate immune effector functions such as ADCC and CDC.